Since the weather turned cold recently my 2001 c240 has been giving me issues. When we have a cold morning neither of my Smart keys will open remotely lock or unlock the doors and then if I open the door manually(withmetal key) and insert the smart key into the iginition it will not allow me to turn the key to position 2. If i wait till temps warm up the smart key will unlock doors and then key will go to second position.
When this issue is happening all interior lights, radio etc DO work. Also to note, when vehicle is cold smart key DOES blink red light when attempting to lock or unlock. Car also does light redlight on shifter (P,R,N) when key is inserted into iginition, so to me that says it sort of acknologizes that it has been inserted but will not do the handshake to allow it to move to second position.

We have checked fuses, checked the battery

I have asked around and some think this may be a bad relay. I would like to check this next. I cannot seem to find a Relay chart or info on where it may be located. I am thinking that it is having an issue that is enhanced by the cold weather.

So does anyone have any info on where this relay might be located? And which it might be, is there a relay that both the iginition process and the electronic remote lock/unlock feature share?

It is a complicated system, I would call MB roadside assitance (free) maybe he can check your battery voltage or attempt to jump it. The key lock/unlock for doors is powered by the batteries in the remote, while the ignition key draws its energy from the vehicle via induction. Your dealer can draw codes, you cant do much, especially if both remotes dont work. I would not rule out a weak battery or poor connection.

Battery was checked it's fine. The car won't work at 8:00 am but then will work at 10:30 when it's warm. I have seen many other users write about things like windows not working or electric seat not working when cold then when sun warms car up they start working again. Apparently there is a Relay somewhere that the sun brings up to a temp that makes it function properly. I just need to figure out what relay controls the door locks and then test it.

This is exactly the problem with my 2001 c240.
Doors open fine (usually) with key.
(Usually) get mechanical noise when key inserted.
However, key will not turn (and engine does not start) until 5-10 min of attempts.
Car is garaged and it is not cold, but after car finally starts is usually no problem for rest of day- only 1st start of day (after no use of car for 12+ hours) is flaky.
Changing batteriesand "rebooting"key byremoving/replacingbatteries in key only improves reliability of unlocking.
Cleaning infrared window on key and socket did not make a difference in starting car.

There have been numerous electrical issues with this car, but one is the one which makes purchasing another MB vehicle a NON-STARTER for me...

I had this same issue on my C280. Did you by any chance get the key wet in anyway?My wife and I where at a 4th of July party and she dropped the key into the pool[:@]. It wokred right after with no issues untill we got home and it died. If youfeel you should get a new kjey thenyou need to go to the dealership or your shop. Give them proof of ownership, drivers license and anything else they ask for. They will call up the place in Germany where they come from and it will in your hands in a week or so. It cost me about $130.00 for the key.

Its not even that... I tried with both my keys.. the keys are fine. we just had a snow storm here and the weather temp dropped a lot... the next day I tried to start the car and it wouldn't... doors wont open car wont do anything at all.